1. Field of the Invention
This invention relates to improved mist lubricant compositions containing specific synthetic esters with a mixture of polyisobutylene polymers having different molecular weights. Synthetic esters employed for the compositions are polyol esters, trimellitate esters, and polymeric fatty acid esters.
2. Description of the Prior Art
Automatic lubrication using mist oils is well known and, for certain applications, recognized as the most effective and economical means of providing a controlled amount of lubricant to the point of lubrication. Mist oil lubrication is particularly useful when the point or area to be lubricated is not readily or safely accessible.
Oil mists are extensively utilized for lubrication of equipment used in steel processing operations. It has been found to be a particularly effective form of lubrication for the bearings of hot roll mills and results in more efficient lubricant utilization and prolonged bearing life. The extended bearing life is generally believed to be the result of (1) more uniform lubricant distribution, (2) lower bearing temperatures, and (3) elimination of contaminants--these latter two benefits being the direct result of the positive air flow associated with the application of the mist to the bearing.
In addition to having acceptable lubrication properties, to be suitable for mist lubrication the oils must also have acceptable mist characteristics. High molecular weight polymers, such as polybutenes, polyisobutylenes, polyacrylates, and ethylene-propylene copolymers, are added to the base oil to develop proper mist characteristics. A general discussion of the effect of polymeric additives on mist properties is presented by T. D. Newingham in Lubrication Engineering, 33 (3), 128-132 (1977).
U.S. Pat. No. 3,510,425 discloses mineral oil compositions useful as mist oils which contain 0.05 to 3.5 weight percent of a polyester. Polyesters which are useful for the formulation of the mist oils have number average molecular weights from 80,000 to 150,000 and are derived from esters of acrylic or methacrylic acid and C.sub.12-12 alkyl monohydric alcohols.
Mineral oil-based mist lubricants are also disclosed in U.S. Pat. No. 3,855,135. Polymeric additives employed for the process of U.S. Pat. No. 3,855,135 have viscosity average molecular weights from 10,000 to 2,000,000 and are selected from polystyrene and polystyrene in admixture with a polyacrylate or polybutene. From 0.01 to 2 weight percent of the polymeric additive is added to the mineral oil.
A process of micro-fog lubrication utilizing mineral lubricating oils containing a minor proportion of a polymeric additive having a number average molecular weight of at least 10,000 is also disclosed in British patent specification No. 1,099,450. The polymeric additives are products which are normally used as VI improvers in motor oils and especially those having low shear stability. Copolymers of vinyl acetate, alkyl fumarate esters and N-vinyl pyrrolidone having number average molecular weights of at least 100,000 are indicated to be particularly useful additives for the process.
U.S. Pat. No. 3,805,918 discloses mist oils containing from 0.001 to 2 weight percent of an oil-soluble polyolefin mist suppressant. Oil-soluble copolymers of ethylene and C.sub.3-12 mono-olefins and having average molecular weights greater than 5,000 are particularly useful additives. In addition to the use of petroleum-derived base oils, hydrocarbon base oils such as alkyl, aryl, and alkaryl phosphate esters, alkyl benzenes, polyoxyalkylene esters or glycols, ortho silicates and siloxanes and also indicated to be useful for the formulation of mist oil compositions employed for the process.
Butene polymers are also utilized to obtain other lubricant compositions. For example, in U.S. Pat. No. 3,098,042 lubricant fluids and greases derived from either mineral or synthetic oils and containing a polymer of butene-1 having a molecular weight in the range 10,000 to 20,000 are disclosed. Various synthetic esters derived from mono- and/or dibasic acids and mono- or polyfunctional alcohols are disclosed as being useful for the preparation of these lubricants. The polybutene-1 can be utilized in an amount from about 0.5 to 12 weight percent. Conventional grease thickeners, such as salts and soaps of fatty acids, may also be present in the composition. Synthetic lubricants with good shear stability and cold temperature fluidity containing 10% to 95% diester with 90% to 5% of a polymer of butene are described in U.S. Pat. No. 3,860,522. The diesters are obtained from branched-chain dicarboxylic acids having from 16 to 22 carbon atoms and aliphatic alcohols having fewer than 6 carbon atoms. The butene polymers have molecular weights from about 1,200 to 4,500. Neither of the above compositions, however, is utilized for oil mist applications.
It would be highly advantageous if oil mist lubricants derived from readily available synthetic ester basestocks and exhibiting improved lubrication and mist characteristics were available.